The supply of fossil hydrocarbons is finite, and eventually will be depleted. Nonetheless, the world economy depends heavily on such hydrocarbons for the production of fuels and petrochemical feedstocks, for example.
It has become apparent that the combustion of fossil fuels may cause irreversible harm to the environment. An example is the increased level of atmospheric carbon dioxide directly traced to the burning of fossil fuels. Studies have shown that increased levels of atmospheric carbon dioxide can have adverse effects on the earth's climate through such mechanisms as "global warming", accompanied by major alterations of weather patterns. Needless to say, an alternative, preferably renewable energy source which would obviate fossil fuels has long been sought.
It is well known that the process of green plant photosynthesis converts light energy into stored chemical energy. In this process, atmospheric carbon dioxide is converted to reduced organic matter such as cellulose, hemicellulose, and lignins. However, biomass is not an ideal form for fuel or chemical feedstock use since it is a solid containing varying levels of water and oxygen. These latter components reduce the fuel value of the biomass.
Various approaches have been undertaken to transform biomass into a fuel or chemical form that is more valuable. For example, gasification and pyrolysis of wood have been described in Organic Chemicals From Biomass, edited by I. S. Goldstein, CRC Press, Boca Raton (1981) (p. 5 for gasification and pp. 63-99 for pyrolysis). In these approaches, biomass is heated in the absence or restricted presence of oxidizing agents such as oxygen, water, and carbon dioxide. In the presence of wood, pyrolysis processes are alternatively referred to as carbonization, wood distillation, or destructive distillation processes. As generally accepted, carbonization refers to processes in which the char is the principal product of interest. Wood distillation refers to liquids produced, whereas destructive distillation refers to both char and liquids. The reaction conditions for gasification and pyrolysis require high temperatures and pressures. Also, under these extreme process conditions, little opportunity is available to control the composition of products. These techniques are basically the same as those used in the processing of coal and yield analogous products.
Instead of subjecting the biomass to reaction conditions which cause simultaneous conversion of all its components, an alternative approach is to choose reactions which selectively convert one component at a time. For example, it is known that cellulose can be converted to glucose by acid hydrolysis. However, acid hydrolysis, in its dilute or concentrated versions is limited in a number of ways. The glucose produced is in aqueous solution, and in dilute acid hydrolysis yields are limited. In concentrated acid hydrolysis, high capital and operating costs are associated with corrosion-resistant equipment and acid recovery and loss.
An alternative method of converting the cellulose fraction of biomass to a fuel is the enzymatic hydrolysis of cellulose. In this method of conversion, an aqueous cellulose suspension is incubated with cellulase enzymes. The enzymatic activation of cellulase causes the hydrolysis of cellulose. The hydrolysis product is glucose. Although this process results in the production of glucose from cellulose, additional process steps such as fermentation and/or water removal are required. Although enzymatic hydrolysis of cellulose can provide 100% yield of glucose, the reaction is much slower than acid hydrolysis, requiring days rather than hours or minutes for completion. Moreover, enzymatic hydrolysis is inhibited by the presence of lignin, which is a major component of most biomass.
U.S. Pat. No. 4,608,137 to Vaughen describes the gasification of carbonaceous material in hydrogen manufacture using divalent iron oxidation as anode reaction and recycled anolyte regenerated by reducing ferric ions with carbon. In this work, hydrogen was manufactured from recycled aqueous electrolyte having pH 3 or less and containing Fe.sup.2+ ions. Fe.sup.2+ is oxidized to Fe.sup.3+ at anode and hydrogen is generated at cathode. U.S. Pat. No. 3,352,773 to Schwartz et al. describes a method of degrading polysaccharides using light radiation and a water-soluble metal or nitrogen based salt of nitrous or hyponitric acid. Cellulosic materials are converted to saccharides of lower molecular weight by irradiation with light.